Two kinds of semi-active laser guided air-frames have been commonly used by the military, one type is the rocket propelled missile and the other type is the howitzer projectile. Each uses a seeker head on the nose of the air-frame to collect laser radiation emitted by the target, and enable guidance. The dish reflector is well suited for the missile seeker while the lens and mirror combination is best for the projectile seeker.
The missile uses a parabolic dish reflector to track the radiation emanating from the target. The dish reflector is thin, light in weight, has a wide aperture, good focusing throughout its field-of-view and allows a compact seeker head antenna. Along with the low weight antenna comes a lighter servo or gyro to point it for tracking. A lighter missile results in greater range.
The cannon launched projectile is subject to high acceleration inside the tube, a much more severe environment than the missile. The thin reflector is not compatible with the high shock resistance required during cannon launch and so it must be stiffened. Nose-heavy means flight stability for a projectile; but the heavier reflector, together with the additional weight of the accompanying servo, translates either into a significant penalty in range, or a significant loss in tracking response needed to follow the radiation.
One of the proposed solutions for the cannon launched projectile has been folded optics. Folded optics affords wide aperture throughout its field-of-view, the shock resistance of a strap-down optical cluster, and a light weight gyro agile enough to track easily. Its reflecting system only works with a flat mirror which is usually polished on the face of the gyro rotor. This is acceptable from a fabrication point of view because making the flat micro surface is an old technology. That said, the folded system is notorious for two troublesome characteristics. One is that the focused image morphs or changes shape as the gyro tracks in pitch or yaw. Focusing this system requires checking its focus throughout its gimbal range. This leads to the second quirk. The image is not plainly visible. These two peculiarities add uncertainty to guidance parameters like gain and feedback and this uncertainty is generally considered a drawback to folded optics. The unpredictability in optical feedback discredits computer flight simulations. The only way to be sure of the projectile's value is the costly way; to build a few and fire them. However, control over optical feedback will make this trait an asset instead of a liability, will bring a substantial improvement to performance and uniformity from one projectile to the next, and will reduce an expensive risk.
Guidance systems that track with a gimbaled antenna in the nose and try to keep a bead on the target have historically been known as using proportional navigation. The mirror is always facing the radiation, even when the missile body turns away from it, and that is the orientation of most concern. The focused spot of light must be centered on a screen in order to indicate when the antenna is on track. Missile body motion can disturb that setting. Optical feedback appears as a second order term in a folded system's transfer function, but this peculiarity is not necessarily bad. It can either enhance or degrade flight stability in a cross-wind or sudden jump in the direction of laser radiation; conditions that typically occur on the battlefield. If feedback has a positive value, then the path of the projectile will spiral away from the target; and that's bad. If it is negative then the projectile will recover from the perturbation and continue to pursue the target; and that's good. This is the reason why precise focusing of the optics is so critical.
Plastic lenses made of polycarbonate are both compact and shock resistant when incorporated into a folded optical system. However the optical characteristics of the plastic lens is sensitive to process variations of molding and annealing, resulting in significant variations in optical characteristics from one lens to another. The lens of most concern is the large plastic objective lens behind the transparent windshield where laser radiation enters. Adjustments must be made for focus for each individual seeker head, because focus, flare and other characteristics are unique to each lens. Quality cannot be held to rigid dimensions or process certification. Each seeker must be focused individually, as the lenses are not interchangeable. This leads to a serious problem.
The focused image in a folded optical seeker head is not plainly visible because it is hidden behind the mirror. This is worth repeating and cannot be over emphasized. The image of the target inside a folded seeker cannot be viewed directly. It cannot be focused by viewing an image and turning a knob, as is the case of a microscope or pair of binoculars. A folded optical seeker is so compact that there is just no way to see inside of it without extraordinary modifications to the system.
Prior to the advent of the present invention, there was no other alternative to focusing each seeker except by indirect means. Focusing done electronically through the output from the photo detector was a long and tedious process requiring skilled technicians, sophisticated equipment, hours of time, and cool precise concentration. The manufacturing record of these systems is speckled with unanticipated delays, loss of schedule and uncontrollable costs. At this writing there is still a need for a measurement system and a method to aid the focusing of seeker heads used in cannon launched projectiles. To date this need has not been satisfied.